1. Field of the Invention
This invention relates to a thick film circuit board, and more particularly to a thick film circuit board with a thick film resistor formed on a silicon carbide substrate board.
2. Description of the Prior Art
Recently a thick film circuit board using a silicon carbide substrate has been proposed (U.S. Pat. No. 4,370,421). When a thick film resistor is directly formed on the substrate by a paste of high resistance (for example, a sheet resistivity of 1 k.OMEGA./.quadrature.) having a glass content of 60% by weight, (a) the thick film resistor swells during firing due to the fact that a carbon dioxide gas generated by reaction with the SiC substrate according to the following reaction is not dissipated, because the paste has a high glass content and the resulting thick film resistor is not porous: EQU SiC+4PbO.fwdarw.SiO.sub.2 +4Pb+CO.sub.2 .uparw.
(b) The thick film resistor has a considerably different coefficient of thermal expansion than that of the SiC substrate, and cracks develop in the thick film resistor.
(c) When the thick film resistor is trimmed by laser, an electroconductive component, which seems to be carbon, is formed at the laser-irradiated parts of the SiC substrate.
On the other hand, when a thick film resistor is directly formed on the substrate with a paste of low resistance (for example, a sheet resistivity of 100 .OMEGA./.quadrature.) having a glass content of 45% by weight, (a) the thick film resistor becomes porous due to the low glass content, and thus the carbon dioxide gas formed by reaction with the SiC substrate during firing is dissipated without swelling of the thick film resistor, (b) the thus formed thick film resistor has the same or similar coefficient of thermal expansion as or to that of the SiC substrate, and no cracks develop, but (c) the electroconductive component is still formed when the thick film resistor is trimmed by laser.